Firefighters from Germany, Romania and Greece were on the ground to help France battle the fire in the Gironde region as well as on other fronts, including in Brittany in the northwest, and more from across Europe were on their way.
The risk of new fires was "very severe" considering the weather conditions, the Gironde prefecture said on Friday.
"The day is likely to be complicated since temperatures continue to increase and humidity continues to drop, so obviously we remain vigilant and mobilised," senior local official Ronan Leaustic told a news conference.
Temperatures as high as 40C were expected in the southwest, with high temperatures also expected across much of France, France's official weather forecast Meteo France said.
The heatwave - officially France's third this summer - was set to ebb on Saturday and end on Sunday with storms, it said.
Some 7400 hectares of forests have gone up in flames in Gironde, and 10,000 people have been forced to leave their homes.
Wildfires have broken out across Europe this summer as successive heatwaves bake the continent and renew the focus on climate change risks to industry and livelihoods.
A huge wildfire raged throughout the night and into its seventh day on Friday in central Portugal, where more than 1600 firefighters backed by nine waterbombing aircraft were combating the blaze that has destroyed about 15 per cent of the Serra da Estrela national park.
After starting in the Covilha area on Saturday, the fire has spread to burn about 15,000 hectares.
Meanwhile, water levels on the river Rhine in Germany have fallen again, with some vessels no longer able to sail, shipping operators and brokers said.
And British households were facing new water usage restrictions, with parts of England likely to declare a drought as the government, environment officials and water companies meet to discuss the prolonged hot and dry weather.
